One of the main problems in aquaculture of marine teleosts is the difficulty of evolving suitable larval diets (Watanabe and Kiron, 1994; Ronnestad et al., 1999). Successful larval rearing through appropriate feeds is a major need and often a critical juncture in closing the life cycle of the candidate species for aquaculture. While live feeds continue to be the main-stay in rearing fish larvae, artificial microdiets are gaining acceptance in the aquaculture industry, but a wholesome artificial feed for fish larvae is not yet available (Watanabe and Kiron, 1994). Indeed there is considerable variation in the larval requirements among fresh-water and marine fishes and crustaceans just as there are differences in the food niches of the adults. As indicated, the changeover from endogenous to exogenous nutrition at the time of exhaustion of internal nutrient supply in the larvae is a critical stage in their life history. This occurs easily in the salmonids, unlike several other teleosts with smaller eggs. The newly hatched salmon larva carries yolk adequate for three weeks’ development, on completion of which the hatchling is ready to accept artificial feeds. In several other fishes the hatchling does not initially have a mouth. The gape of the larval mouth, which is often less than 0.1 mm, decides the size of food accepted by the larvae. The alimentary canal of the newly hatched larvae increases in length and complexity as they grow. In most marine teleosts the newly hatched larvae have no stomach, while the salmonid hatchlings have a stomach before they change to external feeding. By the time the yolk reserves are fully utilized, the feeding capabilities of fish larvae have developed, enabling them to consume feed from external sources (Rosenthal and Alderdice, 1976).
Pelagic fish eggs have up to 50% of the total amino acids as free amino acids (FFAs), which are made available by the hydrolysis of a yolk protein and used for the energy needs of the embryo and larvae. The FFAs reach very low levels at the first feeding of the marine fish larvae after development of their stomachs; the larvae have a low capacity for protein or peptide utilization but can preferentially absorb FFAs. For their energy needs and growth they need high quantities of FFAs in their diets at first feeding. In nature they made this up by preying on plankton, which meet their FFA needs. Hence the need for the right choice of feed organisms/ microdiets at this stage for rearing the post-larvae. In formulated microdiets the amino acids needed to match the amino acid profile of the natural feeds (see for example Kanazawa et al., 1989) have to be made available. This isdifficult because the amino acids often leach out on contact with water. Lopez-Alvarado and Kanazawa (1994) observed that the retention of FFAs in various microdiets tested after a two-minute immersion period in water was dependent on the type of coating. Using liposomes, which are spherical vesicles entrapped by a membrane composed of lipid molecules usually in the form of phospholipids, to enrich live feeds, has been tested successfully (Ozk-izilcik and Chu, 1994). Liposomes have been found to be identical to natural biomembranes. Owing to their compatible size and complete digestibility, liposomes are convenient for the study of nutritional requirements of aquatic filter feeders.
Parker and Selivonchick (1986) used the liposomes successfully in their trials with juvenile Pacific oysters (Crassostrea gigas). The liposomes, however, cannot be directly used as larval microdiets, but can be used to enrich the live food of the larvae, as shown by Ozkizilcik and Chu (1994) who had some success in feeding Artemia with liposomes and enriching the live food with phospholipids and FFAs.
Since the FFAs are an important nutritional component of marine fish larval diets, it is necessary to elucidate further the mechanism involved in the development of digestion and absorption of proteins and amino acids of the early stages of these fish in order to evolve successful larval feeding technologies, and also to match the formulated microdiets as well as the live food organisms used with intestinal capacities (Ronnestad et al., 1999).